1. Field of the Invention
This invention relates to the manufacture of easy open container closures and can end walls and in particular to an improved method and apparatus for scoring metallic container walls.
2. Description of the Prior Art
It is well known to provide a container wall or end closure having a removable or partially removable panel portion which is defined or outlined by a primary score line. The typical end closure has a pull tab attached to the removable panel to rupture the score line and at least partially separate the panel from the end closure. The removable panel may provide a pour opening for beer, beverage or other liquid contents or may be relatively large such as a full panel pull out for food, soup or other solid or semi-solid contents.
It is also well known to provide a secondary score line of lesser depth than the primary score line. This score line, commonly referred to as an "antifracture score" is provided to reduce residual stresses associated with the primary score line so as to prevent or minimize the occurrence of microcracks in or premature fracture along the primary score line. This concept is described in several patents including British Patent 1,164,179 and U.S. Pat. Nos. 3,406,866, 3,735,894 and 3,954,075. The invention disclosed in U.S. Pat. No. 3,954,075 was made by Charles L. Jordan, who is the sole inventor of the present invention.
The disclosure of U.S. Pat. No. 3,954,075 is incorporated herein by reference. That patent illustrates and describes a container wall having a primary score around a removable or partially removable panel or sector and further having a secondary antifracture score on one or both sides of the primary antifracture score. The primary score penetrates the container wall to about one-half its total penetration before the antifracture scores begin to penetrate the metal. That patent states that it is believed that the use of the antifracture score indenters will be beneficial in scoring any bare or unlayered, laminated, layered or coated metal container wall wherein a layer or layers of a material may have different flow characteristics than the metal being scored. It further states that it is believed that application of the compressive force, such as by a secondary indenter, will greatly reduce or obviate tendencies of nonuniform metal flows. The invention of U.S. Pat. No. 3,954,075 was further aimed at facilitating thinner score residuals without cracking of the metal in the primary score line. Thinner score residuals are desirable to reduce the force required to pop and pull the pull tab during opening of an easy open container wall.
In the twenty plus years since the invention of my U.S. Pat. No. 3,954,075, harder temper metals and thinner gauge container sheet have been developed. New laminated and coated container sheet materials have also been developed for use in easy open container walls. End wall designs have also been developed which call for thinner residuals in the primary score line to minimize the force required to initiate rupture and sever the score lines.
Despite the continued mass production of easy open container walls, and the development of higher strength container sheet and coatings/laminates for such uses, the double antifracture score invention of my U.S. Pat. No. 3,954,075 has been utilized in only limited commercial production. The beneficial objectives of that invention have not been realized in commercial production despite the long-standing needs for achieving such objectives. Although the reason for the lack of commercial use of the invention covered by U.S. Pat. No. 3,954,075 is not understood, it is believed that manufacturers of easy open container walls may have experienced problems with failure of the outboard antifracture indenter and/or unsatisfactory reduction of stress corrosion cracking of the metal and/or coatings in the primary score.
There is a need for an improved method and apparatus for scoring metallic container walls to facilitate commercial use of double antifracture scores to minimize or eliminate microcracks of the metal in the primary score line or in a protective coating or layer of organic materials on the underside ("product side") of easy open container walls.